Since microprocessor controlled sewing systems control all machine functions, such systems are normally best suited for operations which require a high degree of operator skill. These operations typically involve sewing a series of seam segments, with the seam margin at the end of each seam segment accurately controlled by the operator. The number of stitches required in each seam can be determined by a constant stored value or, optionally, can be determined by a combination of edge sensing and stitch counting after the sensor detects the material edge. For example, reference is made to application Ser. No. 168,525, filed July 14, 1980, now U.S. Pat. No. 4,359,953 and Ser. No. 210,197, filed Nov. 26, 1980, now U.S. Pat. No. 4,403,558 by applicant.
In order to maximize productivity, an automatic sewing control system must maintain the maximum sewing speed to the last possible moment and then brake and stop as quickly as possible at the end of each individual seam segment without overrunning the end of the seam and with the needle positioned correctly relative to the material. Since the same operation is normally performed many times during the day, the automatic control system can maximize productivity if it can adaptively adjust the braking point to compensate for the current sewing conditions. The invention described herein provides such a capability.
In most sewing operations, the maximum speed of a current machine is in the range of 3,000 to 5,000 stitches per minute. In this speed range, the time required to take an individual stitch is 0.020 second to 0.012 second, respectively. At the end of each seam segment, the position of the needle with respect to the material being sewn is critical. For example, when sewing with a machine employing only one needle, the machine should be stopped with the needle extending into the material in order that the material can be pivoted about the needle when the pressure foot is lifted at the completion of the seam segment. When operating with a machine with two needles, the machine should be stopped with the needles out of the material since it is impossible to pivot about two needles simultaneously. In addition, the vertical position at which the needle on a single needle machine stops is important, since the loop forming mechanism employed by lockstitch sewing machines does not function correctly if the needle is not positioned accurately. The vertical position of a needle conforms to the angular displacement of the motor in a proportional manner. The motor angle at the desired vertical position of the needle is referred to as the "needle positioning angle".
Ideally, it is desirable to maintain the sewing speed on an individual seam segment at the maximum sewing speed and then apply a braking force which decelerates and stops the machine such that the motor angle, at the moment the machine stops, corresponds exactly to the desired needle positioning angle. Otherwise, if the motor angle at the point at which the motor stops is less than the needle positioning angle, additional time is required to rotate the motor until the desired needle positioning angle is achieved. Likewise, if the motor angle at the point at which the machine stops is greater than the needle positioning angle, additional time is required to rotate the motor to form another stitch and then stop at the desired needle positioning angle. In sewing operations which require accurate seam margins, the total number of stitches sewn in each seam must also be accurately controlled.
Prior attempts at stopping a sewing machine at a predetermined needle positioning angle have resulted in the utilization of various velocity control circuits in conjunction with automatic braking circuits. For example, U.S. Pat. No. 3,358,629 issued to Bono discloses a braking circuit that reduces the velocity while the motor angle is sensed by a sector. This sector is correlated or phased relative to the needle so that it functions to stop the machine and corresponds to the needle being at a particular vertical position. When the needle passes the desired vertical position, the braking action is increased such that the needle will stop at a selected position. However, the Bono patent does not disclose controlling the number of stitches within which the motor must stop to accurately control seam margins. U.S. Pat. No. 4,014,277 issued to Morinaga discloses a system which reduces the speed of the motor to a needle positioning speed and detects the position of the needle. When the needle is in the correct position, a brake is applied to stop the motor. With such prior art systems, the speed is decelerated from the maximum sewing speed to a fixed "needle positioning speed" and then maintained at this speed until the desired needle positioning angle is reached. The brake is then reapplied to stop the machine. The prior art does not then disclose controlling the motor angle at which the braking action is initiated in order to minimize the time at needle positioning speed.
On a manually operated sewing machine, the operator can initiate the stopping sequence at any angle during the motor rotation (0.degree. to 360.degree.). Therefore, the motor angle at which the needle positioning speed is achieved can be any motor angle from 0.degree. to 360.degree.. Since the operator applies the brake in a random manner, the average angular difference between the motor angle at which the needle positioning speed is achieved and the desired needle positioning angle will be (statistically) 1/2 of one revolution or 180.degree.. The needle positioning speed on most sewing machines is in the range of 200 to 300 stitches per minute and the time required to take a complete stitch at needle positioning speed is 0.300 second to 0.200 second, respectively. Since, on average, one half-stitch is taken at the needle positioning speed at the end of each seam segment, the time lost per seam segment in a conventional device due to the inconsistent braking point is 0.150 second to 0.100 second, respectively. For a sewing operation requiring four precisely controlled seam segments, this can result in a time loss of 0.600 second to 0.400 second, respectively, each time the operation is performed. Since the operator may perform the operation 2,000 to 3,000 times a day, the resulting total time loss could be in the range of 13.3 minutes to 30 minutes in an eight hour day.
An important aspect of the present invention is to eliminate these time losses to maximize the performancce of microprocessor controlled sewing machines. There currently exists a need for a control system to brake a sewing machine from maximum speed in an optimum manner, such that the motor angle at which needle positioning speed is achieved equals the desired needle positioning angle while taking the correct number of stitches to accurately control seam margins and while accounting for the many parameters which influence the total angular rotation of the motor between the time the brake is applied and the time needle positioning speed is achieved. These factors include the machine speed, inertia, lubrication status, thread size and type, needle size and type, material type, number of stitches being sewn, and numerous other factors.